In the field of implantable medical devices, such as cardiac pacemakers, tachyarrhythmia control devices, implantable drug dispensing devices, monitoring devices, and nerve stimulators, it has become common to provide a transceiver system for performing functions such as the remote programming and the telemetering of data out of the implanted device. For example, in such devices, it has become desirable to have the ability to reprogram the device's modes of operation, parameters, and other functions and/or to monitor the performance of such devices, both historically and contemporaneously. Generally, such implantable medical devices are designed to provide two-way telemetry by radio frequency signal transmission between the implanted medical device and a programming head or wand of an external communication device, e.g., external programmer apparatus, to provide for the exchange of coded transmitted information therebetween.
As the complexity of implantable medical devices increases over time, telemetry systems for enabling such implantable devices to communicate with external communication devices, e.g., programmers, has become more important. For example, it is desirable for a user, e.g., a physician, to noninvasively exercise some amount of control over the implantable medical device, e.g., to turn the device on or off after implantation, to adjust various parameters of the implantable medical device after implantation, etc.
Further, as implantable medical devices include more advanced features, it is typically necessary to convey correspondingly more information to the implantable medical device relating to the selection and control of such advanced features. For example, if a pacemaker is selectively operable in various pacing modes, it is desirable that a physician be able to noninvasively select a mode of operation. Further, for example, if a pacemaker is capable of pacing at various rates, or of delivering stimulating pulses of varying energy levels, it is desirable that the physician be able to select, on a patient-by-patient basis, appropriate values for such variable operational parameters.
Not only has the complexity of implantable medical devices led to the need to convey correspondingly more information to the implantable medical device, but it has also become desirable to enable the implanted medical device to communicate information outside of the patient to an external communication device, e.g., programmer. For example, for diagnostic purposes, it is desirable for the implanted device to be able to communicate information regarding its operational status to the physician. Various implantable medical devices are available which can transmit such information to an external communication device, e.g., the transmission of a digitized ECG signal for display, storage, and/or analysis by the external communication device.
As used herein, the term “uplink” and “uplink telemetry” will be used to denote the communications channel for conveying information from the implanted medical device to an external communication device, e.g., a programmer. Conversely, the term “downlink” and “downlink telemetry” will be used to denote the communications channel for conveying information from an external communication device to the implanted medical device.
Various telemetry systems for providing the necessary communication channels between an external communication device and an implanted medical device have been described. For example, typically, telemetry systems are employed in conjunction with an external programmer/processing unit. A programmer for noninvasively programming a cardiac pacemaker is described in the following U.S. patents to Hartlaub, et al., each commonly assigned to the assignee of the present invention: U.S. Pat. No. 4,250,884, entitled “Apparatus for and Method of Programming the Minimum Energy Threshold for Pacing Pulses to be Applied to a Patient's Heart;” U.S. Pat. No. 4,273,132, entitled “Digital Cardiac Pacemaker with Threshold Margin Check;” U.S. Pat. No. 4,273,133, entitled “Programmable Digital Cardiac Pacemaker with Means to Override Effects of Reed Switch Closure;” U.S. Pat. No. 4,233,985, entitled “Multi-Mode Programmable Digital Cardiac Pacemaker;” U.S. Pat. No. 4,253,466, entitled “Temporary and Permanent Programmable Digital Cardiac Pacemaker;” and U.S. Pat. No. 4,401,120, entitled “Digital Cardiac Pacemaker with Program Acceptance Indicator.” Aspects of the programmer that are the subject of the foregoing Hartlaub et al. patents are described in U.S. Pat. No. 4,208,008 to Smith, entitled “Pacing Generator Programming Apparatus Including Error Detection Means,” and in U.S. Pat. No. 4,236,524 to Powell et al., entitled “Program Testing Apparatus.”
Most commonly, telemetry systems for implantable medical devices employ a radio frequency (RF) transmitter and receiver in the implantable medical device, and a corresponding RF transmitter and receiver in the external communication device, e.g., programming unit. Within the implantable medical device, the transmitter and receiver use an antenna for receiving downlink telemetry signals and for radiating RF signals for uplink telemetry. For example, the radiating RF signals may be magnetically coupled through inductive (antenna) coils.
To communicate digital data using RF telemetry, a digital encoding scheme such as described in U.S. Pat. No. 5,127,404 to Wyborny et al., entitled “Improved Telemetry Format,” is used. In particular, for example, in downlink telemetry a pulse interval modulation scheme may be employed wherein the external communication device, e.g., programmer, transmits a series of short RF “bursts” or pulses in which the duration of an interval between successive pulses, e.g., the interval from the trailing edge of one pulse to the trailing edge of the next pulse, encodes the data. For example, a shorter interval may encode a “0” bit while a longer interval may encode a “1” bit.
The external communication devices, e.g., programming devices, typically interface with the implanted medical device through the use of a programming head or paddle. For example, generally, the programming head or paddle is a hand-held unit adapted to be placed on or near the patient's body over the implant site of the patient's implanted medical device. The programming head may effect closure of a reed switch in the implantable medical device using a magnet to initiate a telemetry session. Thereafter, uplink and downlink communication may take place between the implanted medical device's transmitter/receiver and the receiver/transmitter of the external communication device. Other methods of initiating a telemetry session may also be used. For example, a wake-up pulse from an external communication device may be used to wake-up the implanted medical device, which polls its downlink receiver at an appropriate interval.
For programming arrangements, and/or for monitoring arrangements, both uplink and downlink telemetry signal strength vary as a function of programming head positioning relative to the implantable device. In other words, the signal strength varies as a function of the coefficient of coupling between the communication head, e.g., programming head including an antenna configuration, and the implanted device. Therefore, it is important for the programming head to be properly positioned over the patient's implant site so that downlink RF signals can be detected in the implantable medical device and uplink signals can be detected by the programming head of the external communication device. For example, if the programming head is too far away from the implantable medical device, the attenuation of RF signals transmitted across the boundary of the patient's skin may be too great, preventing a telemetry link from being established.
As such, with appropriate feedback to a user, the user can position and reposition the programming head over the implant site until a suitable position is located to a establish a valid communication link between the external communication device and the implanted medical device. Various feedback techniques have been used to indicate to a user when a programming head has been properly located over a patient's implanted medical device to establish a valid telemetry link.
For example, one technique used for determining when the programming head is properly positioned can be characterized as an “open loop” technique in that the determination of the correct head positioning is based solely upon an assessment of whether the uplink signal (i.e., the signal transmitted from the implanted medical device to the external communication device) meets some minimum requirement. In such an open loop verification system, adequate downlink signal strength is not tested. For example, an open loop system for determining the proper positioning of a programming head is described in U.S. Pat. No. 4,531,523 to Anderson, entitled “Digital Gain Control for the Reception of Telemetry Signals from Implanted Medical Devices.”
A communication protocol using handshaking can also be used to verify that a minimum downlink field strength for detection in the implanted medical device exists to signal a physician that correct head positioning has been achieved. However, conventional handshaking protocols do not provide any information useable for optimization of head positioning to ensure an adequate operating margin. In other words, proper programming head positioning may be indicated even though the programming head is actually marginally positioned, such that a very slight shift in positioning (e.g., due to patient motion) results in downlink telemetry failure.
Further, closed loop systems have also been described for providing feedback to a user for positioning of the communication head for attaining a valid communication link with an implanted medical device. For example, in U.S. Pat. No. 5,324,315 to Grevious, entitled “Closed-Loop Downlink Telemetry and Method for Implantable Medical Device,” a specific type of downlink telemetry pulse is transmitted from the external communication device to the implanted medical device. In particular, the downlink pulses are bursts having a linear ramping envelope. The characteristics of the downlink burst envelope are such that the amplitude of the signal as detected by the implanted medical device's receiver, relative to the receiver's detection threshold, can be ascertained by measuring the time that the detected burst exceeds the receiver's detection threshold. This information can be communicated to the external communication device. In response to receipt of such information regarding the relative strength of the detected downlink signals, the external communication device can modulate the peak amplitude of the downlink burst envelopes by modulating the gain of the external communication device transmitter. As such, the external communication device can then ensure an adequate margin over the implanted medical device's detection threshold while at the same time avoiding the transmission of unnecessarily high energy downlink signals. As described therein, the downlink signal strength and/or the uplink signal strength can be used for activation of a telemetry status indication.
Generally, as described in U.S. Pat. No. 5,324,315, the provision of feedback as to the proper positioning of a programming head with respect to an implanted antenna of an implanted medical device includes the use of a position indicator, for example, an audible tone generator and/or a visible indicator such as a light emitting diode (LED). When signal strength and accuracy are confirmed (e.g., with parity checking, error checking codes, and the like), programmer control circuitry will cause the position indicator to indicate that a link has been established. If adequate signal strength and content accuracy cannot be confirmed, the position indicator will so indicate.
Further, U.S. Pat. No. 5,107,833 to Barsness, entitled “Telemetry Gain Adjustment Algorithm and Signal Strength Indication in a Noisy Environment,” describes provision of a signal strength indicator for providing the user with a visual alpha-numeric readout of signal strength during establishment of a telemetry link. The signal strength is derived from an automatic gain control factor of an adjustable gain amplification stage of an external communication device, e.g., the adjustable gain amplification stage of the uplink receiver of a programmer which receives its input signals from an RF programming head having an antenna configuration therein. The gain of the uplink receiver is a function of the strength of the uplink signal. As described therein, with a telemetry session initiated and uplink signal loss occurring for performing telemetry, the automatic gain control algorithms scan through gain levels searching for one, which will result in valid uplink detection. A displayed range of signal strengths correspond to the scaled automatic gain control levels or factors. Since the automatic gain control value is lowest for maximum signal level and highest for minimum signal level, the value is complemented for use as a signal strength indicator to the user. As described therein, various levels of automatic gain control could be used. For example, scaled values of 0-100, or values of 0-9, may be used for display to a user attempting to position the programming head. The signal strength indicator may appear on a screen of a programmer or it could appear on the programming head as a numeric display for the user to view as the user attempts to find an optimum position for the programmer head based on the viewed strength signal indication.
Further, programmer heads available under the trade designation 9766/9766A/9767, available from Medtronic, Inc., assignee of the present invention, provide for a multiple LED array display for providing indication of proper positioning of the programmer head. The array is driven as a function of the uplink signal strength. The signal strength is determined as a function of the gain of the uplink receiver. A certain number of LEDs of the LED array are activated based upon the signal strength. For example, when the head is not at an optimum position, only one LED may be lit. As the programmer head is moved around the site of the implanted medical device, more LEDs may be lit indicating more optimal positions. Further, no LEDs of the array may be lit until valid telemetry can be accomplished, i.e., as determined by a handshake process.
As described above, conventional RF heads incorporate various types of indicators to guide placement of the RF head. For example, the 9766 family of RF heads available from Medtronic Inc., assignee of the present invention, incorporate signal strength indicator LEDs to guide placement of the RF head. Further, other positioning techniques have used numerical displays for indicating the signal strength to a user to guide placement of the RF programmer head. However, in brightly lit rooms, LEDs or visual numerical displays are sometimes difficult to see and/or read. As such, these indicators are inadequate for optimal placement of an RF programmer head.
Further, in some circumstances, implantable medical devices are implanted in various regions of the body, which prohibit the viewing of an RF programmer head as it is placed over the implant site. For example, neurostimulators are sometimes implanted in the hip area. As such, when a programming head of an external communication device is placed for performing telemetry over the implant site, the user of the head may find it difficult to view LEDs on the head.